When the conventionally shrouded air propeller, such as in an air cushion vehicle, is operating to drive the vehicle forwardly, the air moves through and around the shroud in aerodynamically efficient manner; the relatively blunt sectional form at the leading edge of the shroud preventing "separation" of the incoming air flow. However, upon reverse operation of the propeller the air drawn in through the rear ends of prior type shrouds "separates" as it passes through the relatively sharply sectioned trailing edge portion of the shroud, thereby substantially reducing or perhaps nullifying the effective operation of the propeller.
Air cushion or surface effect type vehicles are typically driven by ducted air propellers as illustrated for example in known U.S. Pat. Nos. 3,241,627; 3,261,420; 3,306,387; 3,331,462; 3,478,836; 3,583,520; 3,608,662; and 3,608,663. The ducts shown therein for shrouding the propellers are designed to provide optimum forward thrust performances, but when such propellers are "reversed" they provide extremely poor reverse thrust effects. Hence, such vehicles are notoriously sluggish in forward motion braking; reverse direction propulsion; as well as variable directional motion control maneuverings, such as are critically important to military and docking operation, and the like.